Technical Field
The present invention relates to magnetic random access memory devices and apparatuses, and more particularly to spin transfer torque cells for magnetic random access memory.
Description of the Related Art
Magnetic random access memory (MRAM) devices differ from conventional random access memory (RAM) in that data is stored through the use of magnetic elements as opposed to storing data through electric charges or current flows. In accordance with MRAM, two magnetic elements are separated by a barrier. In addition, one of the magnetic elements can be a permanent magnet set to a fixed polarity while the polarity of the other magnetic element is adaptable to store data. The different digital states (i.e. one or zero) can be differentiated by assessing whether the polarity of the two elements are the same or different. Data can be read by measuring the electrical resistance of the cell. For example, a transistor can switch a current through the cell such that charge carriers tunnel through the barrier in accordance with the tunnel magneto resistance effect. The resistance of the cell is dependent on the magnetic moments of the two elements. Writing data in an MRAM can be conducted using a variety of methods. Spin transfer torque (STT), which employs a spin polarized current, is one such method.
In accordance with STT, the spin-polarized current is altered as it passes through the adaptable magnetic element, thereby applying a torque to the magnetic element and changing its polarity. Further, there are multiple types of STT MRAM devices. For example, reference layers and free layers of in-plane STT MRAMs have magnetic moments that are parallel to the wafer plane. Alternatively, reference layers and free layers of Perpendicular Magnetic Anisotropy (PMA) STT MRAMs have magnetic moments that are perpendicular to the wafer plane.